Method and apparatus associated with anisotropic shrink in sintered ceramic items

ABSTRACT

A manufacturing method for producing ceramic item from a photocurable ceramic filled material by stereolithography. The method compensates for the anisotropic shrinkage of the item during firing to produce a dimensionally accurate item.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/620,104 filed Oct. 19, 2004, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method for producingceramic items utilizing ceramic stereolithography. More specifically, inone form the invention relates to a method for compensating for theanisotropic shrinkage of a ceramic item to produce dimensionallyaccurate ceramic stereolithography items.

Engineers and scientists are working in the field of stereolithographyto develop additional processes for the production of components. In thearea of non-ceramic stereolithography the scientific community is mainlyconcerned with shrinkage associated with the curing of the polymericmaterial. The types of materials used in non-ceramic stereolithographygenerally have very small shrink rates associated with post cureprocessing; such as by ultraviolet lamps.

In the area of ceramic stereolithography, there presently does notappear to be significant developmental activity going on associated withthe study of dimensional accuracy of sintered ceramic stereolithographyitems. An interest in producing dimensionally accurate parts throughceramic stereolithography provided motivation for the development of thepresent inventions. The present invention satisfies this need and othersin a novel and unobvious way.

SUMMARY OF THE INVENTION

The present inventions are set forth literally in the claims. Theinvention generally can be summarized as a method for compensating forthe anisotropic shrinkage of a ceramic item when it is sintered.

One object of the present invention is to provide a unique method forproducing a ceramic item.

Related objects and advantages of the present invention will be apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative view of one form of an item being fabricatedby a stereolithography process.

FIG. 2 is an illustrative view of the layer built item of FIG. 1.

FIG. 3 is an illustrative top plan view of a portion of one of thelayers of the item of FIG. 2.

FIG. 4 is a perspective view of one embodiment of a shrinkagemeasurement test item.

FIG. 5 is a flow chart illustrating one embodiment of a system forcreating a build file that determines how the item is built.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The general field of ceramic stereolithography is believed known tothose of ordinary skill in the art. More specifically, ceramicstereolithography utilizes a photo-polymerizable resin containingceramic particles that solidifies when exposed to an appropriate energydose. The present invention contemplates that the photo-polymerizabiematerial including ceramic particles can be described in many waysincluding, but not limited to filled and loaded. In one form of thepresent invention the photo-polymerizable material includes ceramicparticles within a range of 35% to 65% by volume; however otherrelationships are contemplated herein.

The photo-polymerizable ceramic resin after being dosed with energyforms a green state ceramic item. The green state ceramic item issubjected to a burning off act to remove the photo-polymer and then asintering act is applied to the ceramic material. During the sinteringof the ceramic material there is a volumetric change in the item.Further, the inventors have recognized that there is generally verylittle volumetric change occurring during the burning off act of thephoto-polymer. In one form ceramic stereolithography is accomplished ina machine adapted for stereolithography operations and available from 3DSystems of Valencia, Calif. However, the present inventions areapplicable with virtually any type of apparatus or techniques forproducing an item by stereolithography. Further, information related toselective laser activation and/or stereolithography is disclosed in U.S.Pat. Nos. 5,256,340, 5,556,590, 5,571,471 and in pending U.S. patentapplication Ser. No. 10/462,168, which are all incorporated herein byreference.

With reference to FIG. 1, there is illustrated one embodiment of an item45 being formed by a ceramic stereolithography process. Ceramicstereolithography as utilized herein should be broadly construed andincludes the utilization of ceramic material within aphoto-polymerizabele resin. The term item is intended to be read broadlyand includes, but is not limited to, molds, parts, components and/orsubcomponents. Item 45 is merely illustrative and is shown being formedby the photo-polymerization of the ceramic filled resin into layers(e.g. 50, 51, 52, 53) of ceramic particles that are held together by apolymer binder. The reader should understand that there is no intentionherein to limit the present application to any particular number oflayers unless specifically provided to the contrary.

Stereolithography apparatus 500 is illustrated in a simplified manner tofacilitate the explanation of one method of making ceramic item 45. Inone form the formation of the layers (e.g. 50-53) utilizes a levelingtechnique to level each of the layers of photo-polymerizable ceramicfilled resin prior to receiving a dose of energy. The presentapplication contemplates the following techniques to level the resin:ultrasonic processing; time delay; and/or mechanically assisted sweepsuch a wiper blade. However, the present application also contemplatesan embodiment that does not utilize express techniques for leveling eachof the layers prior to receiving a dose of energy. A three dimensionalcoordinate system including a first axis, a second axis and a third axisis utilized as a reference for the item being fabricated. In one formthe three dimensional coordinate system is a Cartesian coordinatesystem. More preferably, the Cartesian coordinate system includes an X,Y and Z axis utilized as a reference for the item being fabricatedcorrespond to the axis of the stereolithography apparatus. However,other three dimensional coordinate systems are contemplated herein,including but not limited to polar, cylindrical, spherical. The textwill generally describe the present invention in terms of a Cartesiancoordinate system, however it is understood that it is equallyapplicable to other three dimensional coordinate systems.

In one form stereolithography apparatus 500 includes a fluid/resincontainment reservoir 501, an elevation-changing member 502, and a laser46. The reservoir 501 is filled with a quantity of the photocurableceramic filled resin from which the item 45 is fabricated. Item 45 isillustrated being fabricated in layer by layer fashion in thestereolithography apparatus 500 in the direction of axis Z; which isreferred to as the build direction. The item 45 is built at a buildorientation angle as measured from the axis Z. The build orientationangle illustrated is zero°; however there is no limitation intendedherein regarding the build orientation angle as other build orientationangles are fully contemplated herein. The three dimensional coordinatesystem is aligned with the build orientation angle. More specifically,in a preferred form the three dimensional coordinate system of the itembeing fabricated and the stereolithography apparatus' coordinate systemare coextensive.

With reference to FIG. 2, there is illustrated an enlarged view of aportion of the item 45. The item 45 includes a plurality of cured layers50, 51, 52 which define a portion of the item. The present applicationcontemplates that the term cured includes partially or totally curedlayers. The layers are contemplated as having the same or differentshapes, may be solid or contain voids or holes, may have the same ordiffering thickness as required by the design parameters. In one formthe cured layers have a thickness within a range of about 0.001 to about0.008 inches. In another form each of the layers has a thickness ofabout 0.002 inches. However, other cured layer thickness arecontemplated herein.

With reference to FIG. 3, there is set forth a purely illustrative planview of a portion of a layer 53. Layer 53 represents a portion of alayer formed in a stereolithography apparatus 500 that utilized a wiperblade moved in the direction of axis Y to level the photo-polymerizableceramic filled resin prior to receiving a dose of energy. The wiperblade interacts with the photo-polymerizable ceramic filled material andaffects the homogeneity in at least two dimensions. The inventors havediscovered that the shrinkage in the item associated with a subsequentsintering act is anisotropic in the three directions; for example the X,Y and Z directions. Anisotropic shrinkage can be considered to occurwhen isotropic shrinkage is not sufficient to keep the sintered itemwithin a predetermined geometric tolerance. In the discussion of theanisotropic shrinkage relative to the X, Y and Z axis the Z axisrepresents the build direction and the Y axis represents the directionof the movement of the wiper blade. The inventors have determined thatshrinkage in the Z direction (build direction) is greater than in the Xand Y directions. Factors to consider when evaluating the shrinkage arethe solid loading in the photo-polymerizable resin, the resinformulation, the build style and orientation and how the item issintered.

With reference to FIG. 4, there is illustrated one embodiment of ashrinkage measurement test model 300. In one form the shrinkagemeasurement test model 300 is created as a solid body model and thengenerated as an STL file. In one form the item is oriented such that theback corner represents the origin of a Cartesian coordinate system X, Y,Z. The vertical direction of the STL being aligned with the Z axis andthe two sides 301 and 302 being aligned with the X and Y axisrespectively. The item is than built in a stereolithography apparatuswith the Cartesian coordinate system of the item aligned with thecoordinate system of the stereolithography apparatus. The presentinvention can be utilized with any suitable file format and/or hardware.

The shrinkage measurement test model 300 in the green state is thensubjected to a comprehensive inspection to quantify dimensions of theitem. The measurements taken during inspection can be obtained withknown equipment such as, but not limited to calipers and/or coordinatemeasuring machines. In one form the shrinkage measurement test model hasbeen designed so that all of the inspection dimensions line up along theX, Y and/or Z axis. The item is then subjected to a firing act to burnoff the photo-polymer and sinter the ceramic material. The comprehensiveinspection is repeated to quantify the dimensions of the item afterbeing sintered.

The measured values from the comprehensive inspection after firing arethan compared with the inspection values from the green state item. Inone form the comparison is done by plotting the measured values of thefired item against the measured values from the green state item. Aleast squares analysis is performed to obtain a linear equation. Theresulting slope of the equations is the shrinkage factors for each ofthe X, Y and Z direction/dimensions. The shrinkage for each of the X, Yand Z directions/dimensions are applied to one of the STL file or thesolid body model to expand the dimensions in the respective directionsof the coordinate system. The process will modify one of the STL file orthe solid body model in the directions of the coordinate system toaccount for the anisotropic shrinkage of the item. In one non-limitingexample the shrinkage factors to account for shrinkage are 118%, 115%and 120% in the X, Y, Z direction respectively for an item having alength of about two inches. The present application contemplates a widevariety of shrinkage factors and is not limited in any manner to thesefactors unless specifically provided o the contrary.

The application of the present invention enables the production ofsintered ceramic items having substantially conformity with the item'sdesign parameters. In one form the dimensional accuracy of the sinteredceramic item to the design parameters is within a range of 0.0% to 1.5%and in another form the dimensional accuracy is within a range of 0.0%to 0.5%. Further, the present invention is also applicable to formsintered ceramic items in either near net shape or net shape.Additionally, other degrees of dimensional accuracy are contemplatedherein.

In an alternate form the comparison utilized to calculate the shrinkagefactors of the shrinkage measurement test model is between theinspection values of the fired test model and the dimensional designvalues from the solid body model. The process as described above is thencontinued to find the shrinkage factors for the X, Y and Zdimensions/directions.

With reference to FIG. 5, there is illustrated one non-limitingembodiment of a system for creating a build file 1005 that determineshow the item 45 is created in the stereolithography apparatus. Thisprocess is representative of a technique that can be utilized to producethe build file, but the present application is not intended to belimited to the one embodiment in FIG. 5 unless specifically stated tothe contrary. In act 1000 data defining parameters of the item arecollected and processed to define a specification for the item design.The data from act 1000 is utilized in act 1001 to construct an itemmodel using, for example, a computer modeling system. In one embodimentthe computer modeling system creates an electronic model such as but notlimited to a solid body model. However, other modeling systems arecontemplated herein. The item model from act 1001 is then processed in amodified item model act 1002 to create a model of the item taking intoaccount the anisotropic shrinkage. While the present applicationdiscusses the process in terms of modification of the item model it isunderstood that the same type of modification is applicable to theSTL/STC files to create a modified item file. The modified item act 1002utilizes an X shrinkage factor, a Y shrinkage factor and a Z shrinkagefactor. The shrinkage factors are used to increase the respectiveunderlaying dimensions to a modified dimension. The X, Y and Z shrinkagefactors will be applied so that they correspond to the coordinate systemof the stereolithography apparatus.

In one form a conversion act 1003 is utilized to convert the modifieditem model, produced in act 1002 to a file format, such as STL or SLC.Next, the file from act 1003 is processed in act 1004 to create discretetwo-dimensional slices appropriate for drawing the layers of the itemand any required supports. In act 1005 the build file is completed,which will be utilized to drive the energy source of thestereolithography apparatus and produce the green ceramic item.

In one form the ceramic filled resin comprises a sinterable ceramicmaterial, a photocurable monomer, a photoinitiator and a dispersant. Theceramic filled resin is adapted for use in stereolithography to producea green ceramic item. In one form the filled resin is prepared byadmixing the components to provide a filled resin having viscositywithin a range of about 300 centipoise to about 3,500 centipoise at ashear rate of about 0.4 per second; in another form the filled resin hasa viscosity of about 2,500 centipoise at a shear rate of about 0.4 persecond. However, the present application contemplates filled resinshaving other viscosity values.

The loading of ceramic material within the resin is contemplated withina range of 35% to 65% by volume. Another form of the ceramic loadingwithin the resin is contemplated as being about 50.3% by volume. In onepreferred resin the ceramic loading has the volume percent of ceramicmaterial substantially equal to the weight percent of ceramic materialwithin the resin. However, resins having other ceramic loadings arefully contemplated herein. More specifically, the present applicationcontemplates that the volume percent of the ceramic material in theresin may be equal to the weight percent of the ceramic material in theresin or that the volume percent of the ceramic material in the resinmay be unequal to the weight percent of the ceramic material in theresin. The sinterable ceramic material can be selected from a widevariety of ceramic materials. Specific examples include, but are notlimited to, alumina, yttria, magnesia, silicon nitride, silica andmixtures thereof.

In one example alumina is selected as the sinterable ceramic material.Alumina can be provided as a dry powder having an average particle sizesuitable for sintering to provide an item having the desiredcharacteristics. In one form the powdered alumina has an averageparticle size within a range of 0.1 microns to 5.0 microns. In anotherform the powdered alumina is selected to have an average particle sizewithin a range of 0.5 microns to 1.0 microns. However, other particlesizes for the alumina material are contemplated herein.

The monomer is selected from any suitable monomer that can be induced topolymerize when irradiated in the presence of a photoinitiator. Examplesof monomers include acrylate esters and substituted acrylate esters. Acombination of two or more monomers may be used. Preferably at least oneof the monomers is a multifunctional monomer. By: multifunctionalmonomer it is understood that the monomer includes more than twofunctional moieties capable of forming bonds with a growing polymerchain. Specific examples of monomers that can be used with thisinvention include 1,6-hexanediol diacrylate (HDDA) and 2-phenoxyethylacrylate (POEA). In one form the photocurable monomers are present in anamount between about 10 wt % to about 40 wt %, and in another form about10 wt % to about 35 wt % , and in yet another form about 20 wt % to 35wt % based upon the total weight of the filled resin. However, thepresent application contemplates other amounts of monomers.

The dispersant is provided in an amount suitable to maintain asubstantially uniform colloidal suspension of the alumina in the filledresin. The dispersant can be selected from a wide variety of knownsurfactants. Dispersants contemplated herein include, but are notlimited to, ammonium salts, more preferably tetraalkyl ammonium salts.Examples of dispersants for use in this invention include, but are notlimited to: polyoxypropylene diethyl-2-hydroxyethyl ammonium acetate,and ammonium chloride. In one form the amount of dispersant is betweenabout 1.0 wt % and about 10 wt % based upon the total weight of theceramic within the filled resin. However, the present applicationcontemplates other amounts of dispersants.

The initiator is selected from a number of commercially availablephotoinitiators believed known to those skilled in the art. Thephotoinitiator is selected to be suitable to induce polymerization ofthe desired monomer when irradiated. Typically the selection of aphotoinitiator will be dictated by the wavelength of radiation used toinduce polymerization. Photoinitiators contemplated herein include, butare not limited to benzophenone, trimethyl benzophenone,1-hydroxycyclohexyl phenyl ketone, isopropylthioxanthone,2-methyl-1-[4(methylthio)phenyl]-2-morpholinoprophanone and mixturesthereof. The photoinitiator is added in an amount sufficient topolymerize the monomers when the filled resin is irradiated withradiation of appropriate wavelength. In one form the amount ofphotoinitiator is between about 0.05 wt % and about 5 wt % based uponthe total weight of the monomer within the filled resin. However, otheramounts of photoiniators are contemplated herein.

In an alternate form of the ceramic filled resin a quantity of anonreactive diluent is substituted for a quantity of the monomer. In oneform the amount of substituted nonreactive diluent is equal to betweenabout 5% and about 20% (by weight) of the monomer in the resin. However,the present application contemplates that other amounts of non-reactivediluents are considered herein. An illustration of a given ceramic resincomposition requires 100 grams of a monomer that in the alternate formwill replace about 5-20 wt % of the monomer with a nonreactive diluent(i.e. 95-80 grams of monomer +5-20 grams of nonreactive diluent). Thenonreactive diluent includes but is not limited to a dibasic ester or adecahydronaphthalene. Examples of dibasic esters include dimethylsuccinate, dimethyl glutarate, and dimethyl adipate, which are availablein a pure form or a mixture.

The filled resin is prepared by combining the monomer, the dispersantand the sinterable ceramic to form a homogeneous mixture. Although theorder of addition is not critical to this invention typically, themonomer and the dispersant are combined first and then the sinterableceramic is added. In one form the sinterable ceramic material is addedto the monomer/dispersant combination in increments of about 5 to about20 vol. %. Between each incremental addition of the ceramic material,the resulting mixture is thoroughly mixed by any suitable method, forexample, ball milling for about 5 to about 120 minutes. When all of thesinterable ceramic material has been added, the resulting mixture ismixed for an additional amount of time up to 10 hours or more. Thephotoinitiator is added and blended into the mixture.

With reference to Table I there is set forth one example of an aluminafilled resin. However, the present application is not intended to belimited to the specific composition set forth below unless specificallystated to the contrary.

wt/g vol cc wt % vol % Alumina 1980 500 78.2 48.0 Monomer 510 500 20.148.0 Dispersant 39.6 38.8 1.56 3.73 Photoinitiator 2.55 2.32 0.101 0.223Total 2532 1041 100% 100%

In one form the green ceramic item is sintered to a temperature within arange of 1100° C. to 1700° C. The present invention contemplates othersintering parameters. Further, the present invention contemplatessintering to a variety of theoretical densities, including but notlimited to about 60% of theoretical density. The density of the sinteredmaterial is preferably greater than sixty percent of the theoreticaldensity, and densities equal to or greater than about ninety-fourpercent of the theoretical density are more preferred. However, thepresent invention contemplates other densities.

The present application contemplates the utilization of a threedimensional coordinate system as a reference for an item beingfabricated from the photo-polymerizable ceramic filled resin. Asdiscussed above the inventors have discovered that the shrinkage of theitem in a subsequent sintering act is anisotropic in the threedirections. Therefore, in one form of the present invention there areutilized three unequal scaling factors to take into consideration therespective shrinkage in the dimensions of the item in all threedirections. In another form of the present invention there are utilizedonly two unequal scaling factors to account for the respective shrinkagein the dimensions of the item in all three directions; that is thedimensions in two of the three directions are adjusted by scalingfactors having the same value.

While the inventions have been illustrated and described in detail inthe drawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. It should be understood that while the useof the word preferable, preferably or preferred in the description aboveindicates that the feature so described may be more desirable, itnonetheless may not be necessary and embodiments lacking the same may becontemplated as within the scope of the invention, that scope beingdefined by the claims that follow. In reading the claims it is intendedthat when words such as “a,” “an,” “at least one,” “at least a portion”are used there is no intention to limit the claim to only one itemunless specifically stated to the contrary in the claim. Further, whenthe language “at least a portion” and/or “a portion” is used the itemmay include a portion and/or the entire item unless specifically statedto the contrary.

1.-56. (canceled)
 57. A method, comprising: representing an itemincluding its dimensions relative to a three dimensional coordinatesystem, the item adapted to be formed as a green ceramic item in astereolithography apparatus from a photo-polymerizable materialincluding ceramic particles; applying three unequal scaling factors tothe dimensions to compensate for the anisotropic shrinkage of the greenceramic item relative to the axes of the coordinate system upon thegreen ceramic item being sintered; building the green ceramic item inthe stereolithography apparatus after said applying; and heating thegreen ceramic item to form a sintered ceramic item substantiallyconforming to the dimensions.
 58. The method of claim 57, furthercomprising applying a largest scaling value to one of the unequalscaling factors corresponding to a build direction.
 59. The method ofclaim 57, wherein said building includes forming a plurality of layersof the photo-polymerizable material; and which further includes levelingat least one of the plurality of layers of the photo-polymerizablematerial prior to forming the next of the plurality of layers of thephoto-polymerizable material; and which further includes exposing the atleast one of the plurality of layers of the photo-polymerizable materialwith a dose of energy prior to forming the next of the plurality oflayers of the photo-polymerizable material.
 60. The method of claim 59,wherein said leveling including mechanically wiping a surface of the atleast one of the plurality of layers of the photo-polymerizablematerial.
 61. The method of claim 60, wherein said mechanically wipingchanges the homogeneity of the ceramic particles in at least twodimensions.
 62. The method of claim 60, wherein said leveling includesleveling each of the plurality of layers of the photo-polymerizablematerial; and which further includes exposing each of the plurality oflayers of the photo-polymerizable material with a dose of energy. 63.The method of claim 57, wherein the stereolothography apparatus includesa machine three dimensional coordinate system coextensive with the threedimensional coordinate system.
 64. The method of claim 57, wherein thesintered ceramic item is of a near net shape.
 65. The method of claim57, wherein the sintered ceramic item is of a net shape.
 66. The methodof claim 57, wherein the sintered ceramic item is dimensionally accuratewith the dimensions within about 1.5%.
 67. The method of claim 57,wherein the sintered ceramic item is dimensionally accurate with thedimensions within about 0.5%.
 68. The method of claim 57, wherein saidheating includes burning off a polymer in the green ceramic item. 69.The method of claim 58, wherein the three dimensional coordinate systemis a Cartesian coordinate system including a first axis corresponding tothe X axis and a second axis corresponding to the Y axis and a thirdaxis corresponding to the Z axis; wherein the stereolothographyapparatus includes a machine three dimensional coordinate systemcoextensive with the three dimensional coordinate system; which furtherincludes selecting a build direction corresponding to the Z axis;wherein said building includes forming a plurality of layers from thephoto-polymerizable material in the build direction; and which furtherincludes wiping an upper surface of each of the plurality of layers ofthe photo-polymerizable material prior to being exposed to an energydose, said wiping in the direction of one of the other axes.
 70. Themethod of claim 57, wherein the photo-polymerizable material includesceramic particles within a range of 35% to 65% by volume.
 71. The methodof claim 57, wherein said representing is associated with any ofelectronic data, digital data, virtual data, computer files, solid bodymodeling, computer modeling, computer aided manufacturing.
 72. Themethod of claim 57, wherein said heating is to a temperature within arange of about 1100° C. to 1700° C.
 73. A method, comprising: definingthe original dimensions of an item with reference to a three dimensionalcoordinate system associated with a stereolithography apparatus, thecoordinate system including a first direction, a second direction, and athird direction; applying a first factor to increase the originaldimensions of the item defined relative to the first direction; applyinga second factor to increase the original dimensions of the item definedrelative to the second direction; applying a third factor to increasethe original dimensions of the item defined relative to the thirddirection; wherein the third direction corresponds to a build direction,and wherein the factors are not equal; building a green ceramic item bystereolithography from a photo-polymerizable resin including ceramicparticles, said building utilizing the increased dimensions from saidapplying acts to control the dimensions of the green ceramic item; andsintering the green ceramic item to form a sintered ceramic item,wherein the sintered ceramic item has dimensions correspondingsubstantially to the original dimensions of the item.
 74. The method ofclaim 73, wherein said third factor is a largest one of the factors. 75.The method of claim 73, wherein the sintered ceramic item isdimensionally accurate with the dimensions within about 1.5%.
 76. Themethod of claim 73, wherein the sintered ceramic item is dimensionallyaccurate with the dimensions within about 0.5%.